The layout of the NA62 beamline and detector [1]} is shown schematically in Fig. REF . An unseparated secondary beam of \(\pi ^+\)  (70%), protons (23%) and \(K^+\)  (6%) is created by directing 400 GeV/\(c\)  protons extracted from the CERN SPS onto a beryllium target in spills of 3 s effective duration. The central beam momentum is 75 GeV/\(c\) , with a momentum spread of 1% (rms).
